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ang1dust 04-20-2013 10:04 PM

Advice on filters
This isnt your normal advice request. First i got this idea from inspecting my filter. It is 25x25 and the fabric of the filter is an accordian style and i came to realize that they do that on purpose. From basic logic i beleive the accordian is intended to increase the amount of air flow. A straight across filter would not be as efficient. So i got this idea , weather that is actually true or not, i want to take my 25x25 plenum and fold it in half. Stay with me. I want to put 2 20x25's inside the plenum at a slant in a 90 degree fasion in the plenum. This way instead of getting 1 25x25 air flow of 1250 i can put 2 20x25's at whatever their cfm is.

My reason for this is:
I currently run a 4 ton unit.
I have 1 main 25x25 intake (1250 cfm total) which max for a 4 ton is 1600 cfm iirc
So if i make this filter setup a reality it will max out my intake decreasing the strain on my air conditioner and the output it can cool.

Let me know what you think.

HVAC1000 04-21-2013 01:06 AM

i think thats a pretty good idea. a lot of new construction installers dont supply enough return air to an air handler and then on top of that the filter restricts the flow even more. it is never a bad idea to get more filtering surface area. make sure you can make the proper sheet metal parts to properly support the filters and keep them in place. good luck

yuri 04-21-2013 07:50 AM

properly is the key word. even a 1/4 inch gap will allow LOTS of dust to get by. need to make the supports from sheet metal not just some wood sticks. HDepot has all kinds of neat aluminum and metal angle material and channels in the hardware area, some aluminum types too which would be OK.

ang1dust 04-21-2013 12:39 PM

Yes, i was going to use just 1x2 wood "sticks" as you call it but after the last post i thought abou tit and the metal suggestion reminded me of the wide variety of items they have, I will make sure its a good seal.

But i must ask the meaning of this post is to question the validity of my project. A 25x25 (reduced by the actual hosing to an 18 inch hose) for return can take in 1400-1600 cfm iirc. A 25x25 can take in approx 1250 cfm clean..

does this project seem to be legitimate? Does anyone have any proof of what im saying as providing more retrun cfm?

I mean that is why they slant some of these air returns in apartment complexes right? So they can fit a smaller return hole but still have the same type of filtered return?

I just want to make sure im not waisting my time by doing this project. Just want someone to tell me "yes, it will significantly change the amount of return air" lol :)

Also, i just thought of this, will it make a difference if the arrow, the point of the filters are pointed up or down? Extrovert vs introvert in terms of making the best air flow?

beenthere 04-21-2013 03:12 PM

If your putting this V configuration in the duct system at the furnace/air handler. It could reduce air flow.

HVAC1000 04-21-2013 04:05 PM

How many btu tonnage and cfm of your unit? Also is your return fed into the bottom or side of the unit. Maybe some pictures?

ang1dust 04-21-2013 07:21 PM

I can do pictures tomorrow. The intent of the V or ^ shape of the is to increase air flow not restrict. Thats my main question. If my 1 filter is intended to move 1250 cfm, if i change it to a V shape inside my plenum whats the possibilit of its success to increase filtered air flow? Low medium high? how much? a little some or alot?

I figure the V shape will obviously impede aire in the middle where the 2 filters come together but if i have two filters 20x25 in a V fasion inside a plenum 25x25 (or larger thats just the size of the grill for the current filter) it should offere a decent 40% increase in air flow yes?
It will only pull 1400-1600 to begin with because it connects to an 18 inch return but i just want to max it out if possible.

beenthere 04-21-2013 07:48 PM

Your 25X25 air filter will allow 1600 CFM through it at 410 FPM. So I don't know where you are coming up with only 1250 CFM. You aren't talking about a return air filer grille are you.

ang1dust 04-21-2013 10:26 PM


Originally Posted by beenthere (Post 1164550)
Your 25X25 air filter will allow 1600 CFM through it at 410 FPM. So I don't know where you are coming up with only 1250 CFM. You aren't talking about a return air filer grille are you.

No, a few months ago i did a lot of research that showed that a 25x25 filter would only yeild 1250 cfm. Am i wrong? Is that also to say 18 inch return will yeild more than 1600?

beenthere 04-22-2013 04:16 AM


Originally Posted by ang1dust (Post 1164667)
No, a few months ago i did a lot of research that showed that a 25x25 filter would only yeild 1250 cfm. Am i wrong? Is that also to say 18 inch return will yeild more than 1600?

What FPM was used on the filter that your research showed only 1250 CFM? An 18" can move more then 1600 CFM.

If a 25X25 could only allow 1250 CFM through it. Your 4 ton A/C would freeze up every time it runs. Does it?

ang1dust 04-23-2013 06:35 PM

no it does not freeze up

I have an 18 inch return connected to a 25x25 grille (probably a 28x28 plenum)

For starters it is a half breed 4 ton system slowly converted from 3 ton. It was 3 ton when built and over time as things were replaced it was all upgraded to a 4 ton unit but my return and supply stayed the same. So i am attempting to increase my return in hopes of a relief of the system since the house is set up for a 3ton. Ive been told by a few people that an 18 inch is not enough for a 4 ton system. Furthermore, a 25x25 filtered doesn't return enough to max out an 18 inch return. So this is where i figured i had a problem. My system does not cool well at all. I live in texas and its a 2 story and i am doing whatever i can to get the most out of my struggling system.

Right now i have to leave my thermostat on 84 degree to keep from having it come on at 2pm and not go off untill 7pm....otherwise if it is on all day long it simply depletes my checking account :(

beenthere 04-23-2013 06:59 PM

While an under sized return is often the problem with a system. You probably have far more trouble from an under sized supply system.

An 18" flex duct 100 foot long can move 1600 CFM at a friction rate of .09". Is your return duct 100 foot long?

A Hart&Cooley 659 return air grille 24x24 is listed to move 1665 CFM at 600 FPM. While that is too fast for a return grille. It can move the 1600 CFM you want. Instead of just increasing return duct and grille size. it probably ill work better to add another return. then improve your supply ducts. how many supplies do you have, and what size are they.

ang1dust 04-23-2013 10:04 PM

I have too much supply is 1 problem. From calculations i have over 2200cfm supply :(
i have
4 - 8 inch supply for bedrooms
1 - 6 inch for stair well
3 - 4 inch supply for 3 bathrooms
1 - 22x14 plenum going down to
2 - 8 inch split for the kitchen
1 - 8 inch for the dining room
1 - 8x11 inch for the living room (this is set up to be 8x16 wide but i cut it down becuse i ws getting no cooling up stairs and i put a 5 inch peice of sheet rock to cut it down but it blows about 25mph (i dont know the equivilnt of fpm) so downstiars gets so cold so fast) But it doesnt get cold nearly as bad during the day...If its over 92ish degrees outside i have to set my thermostat to 80, if its abour 98 it needs to be about 82, any higher it should be about 84 otherwise my AC runs for HOURS trying to get it down just half a degree....

so a quick explanation of this setup to give a visual
2 bedroom house, built on a square
there is a center to this house that it is bluit around so you can walk in a circle on both levels

the HVAC is in attic, it distributes horozontally to an T fashioned plenum that pushes air to the upstairs through an L fashion but the upstairs makes it a T pushing 4 8" and 2 4". This is all in the Attic and its return is upstairs and is about 90 feet long.

did i miss any details?

jagans 04-24-2013 09:36 AM

Why don't you just use a deeper filter with more surface area, or a less restrictive filter, and change it more often? Seems to me the V would set up more turbulence and restriction to some degree.

tylernt 04-24-2013 10:16 AM


Originally Posted by ang1dust (Post 1165991)
otherwise my AC runs for HOURS trying to get it down just half a degree....

That sounds to me like a symptom of too few tons of cooling?

Anyway, V filters seems like a fine idea to me, because both the furnace+A/C in my last house, and now my furnace-only in my current house, had a V setup from the day they were installed... I thought it was normal.

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